Sensor for detecting defects in a component

ABSTRACT

A sensor for detecting defects in a component includes an electric coil fed with a varying electrical current to create a varying magnetic field penetrating at least partly into the component under test, and a defect detector including a magnetoresistor. The above components are accommodated in a protective housing having a detection face disposed near and parallel to a surface of the component under test. The coil has its axis Δ perpendicular to the detection face and the magnetoresistor is in the vicinity of the detection face. The magnetoresistor is a giant magnetoresistor and is disposed so that its sensitivity axis Δ 1  sensitive to variations in a magnetic field is parallel to the detection face of the housing. The sensor further includes a first permanent magnet disposed so that it magnetically biases the magnetoresistor in the direction of its sensitive axis Δ 1  to a value such that the operating point is a point on a curve, representing the output signal of the magnetoresistor as a function of the value of the component of the magnetic field in the direction of the sensitive axis, which is situated in the vicinity of the middle of a substantially rectilinear portion of the curve.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a sensor for detecting defectsin a component.

[0003] The component to be tested can be electrically conductive andferromagnetic or non-ferromagnetic, and in this case the invention candetect defects such as surface or internal cracks.

[0004] The invention applies equally to detecting electricallyconductive ferromagnetic or non-ferromagnetic particles in a materialthat is not electrically conductive.

[0005] 2. Description of the Prior Art

[0006] Various techniques for non-destructive testing of metalcomponents are known in the art: for example, Eddy current testing bymeasuring the impedance of sensing coils, or testing ferromagneticmaterials by the dispersion flux method using magnetoresistive sensors.The document EP 0 736 173 B1 describes, with reference to the associatedFIG. 2, a sensor including an excitation coil 21 whose axis isperpendicular to the surface of the product under test and which is fedwith a varying electrical current to generate Eddy currents in theproduct.

[0007] The detector means include two Eddy current detector coils 22mounted in opposition and two magnetoresistors 23 disposed parallel toeach other and connected in a differential circuit. The Eddy currentdetector coils are coaxial with and inside the excitation coil 21 andthe magnetoresistors 23 are parallel to the axial direction of thecoils. The detector coils and the magnetoresistors are connected toprocessor means that include a multichannel processor circuit.

[0008] An object of the present invention is to propose a differentsensor arrangement offering very high sensitivity.

SUMMARY OF THE INVENTION

[0009] The invention therefore provides a sensor for detecting defectsin a component, the sensor including an electric coil fed with a varyingelectrical current to create a varying magnetic field penetrating atleast partly into the component under test, and defect detector meansincluding a giant magnetoresistor, the above components beingaccommodated in a protective housing having a detection face adapted tobe disposed near and parallel to a surface of the component under test,the coil having its axis Δ perpendicular to the detection face, and thegiant magnetoresistor being situated in the vicinity of the detectionface and disposed so that its sensitivity axis Δ1 sensitive tovariations in a magnetic field is parallel to the detection face of thehousing, which sensor further includes a first permanent magnet disposedso that it magnetically biases the giant magnetoresistor in thedirection of its sensitive axis Δ1 to a value such that the operatingpoint is a point on a curve, representing the output signal of the giantmagnetoresistor as a function of the value of the component of themagnetic field in the direction of the sensitive axis, which is situatedin the vicinity of the middle of a substantially rectilinear portion ofthe curve.

[0010] If the component under test is made of a ferromagnetic material,the sensor advantageously further includes a second permanent magnetassociated with an open magnetic circuit having two end surfaces ofopposite polarity and situated against the detection face and ondiametrally opposite sides of the electrical coil.

[0011] One embodiment of the invention will now be described withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a sensor in accordance with the invention which isparticularly suitable for testing amagnetic material components.

[0013]FIG. 2 shows a sensor according to the invention that isparticularly suitable for testing ferromagnetic material components.

[0014]FIG. 3 is a very simplified functional block diagram of a devicefor detecting defects.

[0015]FIG. 4 is a curve of the output voltage V of a giantmagnetoresistor as a function of the value H of the component of themagnetic field at the location of the magnetoresistor and in thedirection of the sensitivity axis of the magnetoresistor, and theoperating point on that curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] Referring to FIG. 1, which shows a sensor 1 according to theinvention, a housing 2 that is made of a material that is notelectrically conductive and has a bottom face 3 referred to as thedetector face contains an electrical coil 4 whose axis Δ isperpendicular to the detection face 3.

[0017] The coil 4 is fed with a varying electrical current to create avarying magnetic field penetrating at least partly into the component 5under test, along which the sensor 1 is moved.

[0018] Two giant magnetoresistors (GMR) 6 and 7 are located near thedetection face 3, inside the coil 4 and close to its bottom end.

[0019] A GMR is a directional component that is sensitive only to thecomponent of the magnetic field in the direction of its sensitivityaxis. A GMR is very sensitive to variations in the field in thatdirection.

[0020] The GMR 6 and 7 are disposed side by side so that theirsensitivity axes are parallel to the detection face 3. The direction ofthe sensitivity axis is represented by the line Δ1.

[0021] A single GMR could be used, but using two and associating them ina differential circuit eliminates causes of errors due to spurious fieldvariations at the location of the GMR when the sensor moves along thecomponent 5. For example, this eliminates most of the effect of theterrestrial magnetic field that would otherwise cause a measurementerror if the path of the sensor were not rectilinear, for example.

[0022]FIG. 4 shows the response curve R, R′ of the output voltage V of aGMR as a function of the value of the field H surrounding it (only thevalue of the component of the field H in the direction of thesensitivity axis of the GMR is considered). As this figure shows, thecurve is symmetrical with respect to the ordinate axis V, i.e. theresponse is positive regardless of the direction of the field along thesensitivity axis.

[0023] Referring again to FIG. 1, the sensor further includes apermanent magnet 8 disposed above the coil 4, i.e. on the other side ofthe coil relative to its face placed against the detection face 3.

[0024] It is placed on the axis of the coil at the end of a threaded rod9 which screws into a non-conductive and amagnetic support 18 with ascrewthreaded hole through it enabling the position of the magnet alongthe axis Δ of the coil to be adjusted. A nut 10 locks the rod in theadjusted position.

[0025] The position of the magnet 8 is adjusted so that, at the locationof the GMR 6 and 7, it magnetically biases the sensitive axis Δ1 of theGMR to a value such that the operating point is at a point 11 on thecurve R in FIG. 4 in the vicinity of the middle of the substantiallyrectilinear portion of the curve.

[0026] Thus, in the absence of defects in the component 5, the resultantfield Hr in the direction of the sensitivity axis Δ1 oscillates betweentwo values H₁ and H₂ corresponding to a rectilinear part of the curve Rand situated outside the bottom area, in which there is hysteresis, andbefore saturation occurs.

[0027] The sensor 1 also accommodates a printed circuit card 13 carryingthe components of the differential circuit including the GMR 6 and 7,together with the necessary matching circuit. A connector 14 connectsthis card to the power supply of the coil 4 and the GMR 6 and 7.

[0028] The connector 14 is also connected to an Eddy current generator15 (FIG. 3) which supplies the coil 4 and the GMR 6 and 7 and processesthe output signal of the differential circuit 16 of the GMR and thematching circuits 17 on the printed circuit card 13.

[0029] The sensor shown in FIG. 1 is particularly suitable for testingnon-ferromagnetic conductive material components 5.

[0030] The coil 4, energized with a varying current, induces in thecomponent 5 Eddy currents which in turn produce a varying magneticfield, which generates a resultant field Hr at the location of the GMR(Hr is the component of the field in the direction of the sensitive axisΔ1 of the GMR).

[0031] If the sensor encounters a defect such as a crack, the Eddycurrents are diverted and this modifies the field produced by thosecurrents and therefore the resultant field Hr, which modification istherefore detected.

[0032] The Eddy current generator 15 can modify the excitation frequencyof the coil 4 to scan for defects at varying depths in the component 5.

[0033] The sensor can equally well be used to detect ferromagnetic ornon-ferromagnetic conductive particles in a non-conductive component 5:when the sensor passes over such particles, the resultant magnetic fieldHr is modified and this is detected.

[0034]FIG. 2 shows a sensor according to the invention which isparticularly suitable for identifying defects in ferromagnetic materialcomponents 5.

[0035] To this end, in addition to the excitation coil 4 and thepermanent magnet 8 for biasing the GMR 6 and 7, as in FIG. 1, the sensor100 further includes means for locally magnetizing the component 5 undertest so that all the zones scanned by the sensor 100 have a constantrelative magnetic permeability μr such that visible defects caused infact by lack of magnetic homogeneity of the component 5 are notdetected.

[0036] To be able to detect defects deep inside the component, it isdesirable to magnetize the component 5 relatively strongly, to obtain apermeability μr close to 1.

[0037] To do this, the support 18 of the sensor 1 is replaced by an openmagnetic circuit 19 including at least one permanent magnet; in FIG. 2there are two permanent magnets 20 and 21.

[0038] The magnetic circuit 19 includes two polepieces 22 and 23 whoseend surfaces of opposite polarity are situated against the detectionsurface 3, on diametrally opposite sides of the coil 4 in the directionof the axis Δ1.

[0039] As a general rule, the magnetic circuit is disposed so that thefield lines extending from one polepiece to the other all pass into thecomponent 5 without passing through the GMR 6 and 7.

[0040] The magnetic circuit 19 is also used as a support for the magnet8 and its threaded support rod 9 screwed a greater or lesser distanceinto a screwthreaded hole in the magnetic circuit 19. The locking nut 10is also shown. As in FIG. 1, there is a printed circuit card 13 carryingthe circuits 16 and 17 from FIG. 3, providing the same functions, andthe connector 14.

[0041] As in FIG. 1, in the event of defects, such as a crack, in theferromagnetic component 5 from FIG. 2, the Eddy currents produced in thecomponent 5 by the coil 4 are diverted and produce a different inducedfield, modifying the resultant field Hr at the location of the GMR 6 and7.

There is claimed:
 1. A sensor for detecting defects in a component, saidsensor including an electric coil fed with a varying electrical currentto create a varying magnetic field penetrating at least partly into saidcomponent under test, and defect detector means including a giantmagnetoresistor, said components being accommodated in a protectivehousing having a detection face adapted to be disposed near and parallelto a surface of said component under test, said coil having its axis Δperpendicular to said detection face, and said magnetoresistor beingsituated in the vicinity of said detection face and disposed so that itssensitivity axis Δ1 sensitive to variations in a magnetic field isparallel to said detection face of said housing, which sensor furtherincludes a first permanent magnet disposed so that it magneticallybiases said giant magnetoresistor in the direction of its sensitive axisΔ1 to a value such that the operating point is a point on a curve,representing the output signal of said giant magnetoresistor as afunction of the value of the component of said magnetic field in thedirection of said sensitive axis, which is situated in the vicinity ofthe middle of a substantially rectilinear portion of said curve.
 2. Thesensor claimed in claim 1, for detecting defects in a ferromagneticcomponent, further including a second permanent magnet associated withan open magnetic circuit having two end surfaces of opposite polarityand situated against said detection face and on diametrally oppositesides of said electrical coil.
 3. The sensor claimed in claim 1,including two giant magnetoresistors in a differential circuit.
 4. Thesensor claimed in claim 1, wherein said first permanent magnet issituated over said electrical coil, on the opposite side of said coilrelative to its face situated against said detection face, and saidsensor includes means for adjusting its position along the axis of saidcoil.